Move to pkg-config

[Clement Pernet]

Hi,

A few months ago, we have introduced pkg-config files (*.pc) to have linbox, fflas-ffpack and givaro
export their compilation flags in a simple uniform and easy to use way:

once installed, you simply need to run
pkg-config linbox --cflags
to get the compilation flags of linbox
pkg-config fflas-ffpack --libs
to get the linking flags of fflas-ffpack.

This is very similar to the linbox-config.sh scripts that we are also shipping, but the pkg config
system is becoming standard and is for example used extensively in SageMath to configure the lib
dependencies.

As a second step, I changed the way the dependencies of LinBox to fflas-ffpack and fflas-ffpack to
givaro and handled, by disabling the former m4 scripts, performing compilation based tests, and
replacing them with the simpler and faster PKG_CHECK_MODULES macro.

This change may result in a failure to configure on your system for the 2 following cases:

1/ you do not have pkg-config install on your system.
apt-get install pkgconf
should solve it.
2/ you have installed the dependency (say givaro) to a specific folder (outside /usr/local) and
pkg-config fails to find the givaro.pc file
you should update the search path of pkg-config with something like
export PKG_CONFIG_PATH=${PKG_CONFIG_PATH}:<dependency-prefix>/lib/pkgconfig

Please let me know by replying to this thread if you have any problem with this change.

Best

Clément